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PubMed1980

Evidence that the human jaw stretch reflex increases the resistance of the mandible to small displacements.

1. Small 'step' or sinusoidal displacements were imposed on the mandible while human subjects maintained an average biting force of 10 N. Phase-related changes in the force resisting sinusoidal displacement were used to determine the mechanical stiffness of the human mandibular system as a function of the frequency of stretching. 2. Jaw-muscle electromyographic (e.m.g.) responses to 'step' stretches were of 8 msec latency and generated a very substantial force response. Jaw-muscle e.m.g. responses having longer latency were not observed. 3. The mechanical stiffness of the human mandible was relatively constant as a function of the frequency of stretching, having a typical magnitude of about 15 N/mm (+/- 200 micrometers stretch) or 10 N/mm (+/- 1500 micrometers stretch) at mean biting forces of 10 N. The force resisting displacement was phase-advanced at all frequencies. 4. Modulation of jaw muscle electrical activity evoked by sinusoidal stretches increased in amplitude as a function of increasing stretch frequency. E.m.g. modulation was 60--100 degrees advanced at frequencies of 1--10 Hz, but the phase decreased at higher frequencies, becoming negative (lagging stretch) at frequencies of 30 Hz and above. These characteristics are consistent with the idea that the jaw stretch reflex is dependent on jaw muscle spindle afferent fibres exciting jaw-closing motoneurones by relatively direct (but not necessarily monosynaptic) connexions. 5. The relationship between jaw-muscle activity and voluntary fluctuations of isometric biting force suggests that human jaw muscles can be modelled as a second-order linear 'filter'. The corner frequency for human jaw muscle is about 3 Hz; thus it would appear to be considerably slower than jaw muscle of monkeys. 6. The reflex stiffness of the human mandible, estimated quantitatively on the assumption that human jaw muscle stiffness is similar to the intrinsic stiffness of the gastrocnemius of the cat, ranges between 5 and 9 N/mm at frequencies between 1 and 8 Hz. Since this reflex stiffness is about the same as muscle stiffness in this frequency range, we conclude that the stretch reflex of the human mandible contributes functionally to its postural stability. 7. Reflex stiffness appears to be greater in the monkey mandible relative to muscle stiffness than in the human mandible. The difference is argued to be a manifestation of the difference in jaw muscle contraction speed between the two species. 8. The fact that the mandibular stretch reflex appears to be stronger than the stretch reflex of the limbs of intact animals and humans is discussed in terms of the special anatomical and functional features of the mandible.

Adult↗
PubMed1981

Response of soleus Ia afferents to vibration in the presence of the tonic vibration reflex in the decerebrate cat.

1. Micro-electrode recordings were made from single Ia afferents in the intact nerve to the soleus muscle in the decerebrate cat while the muscle was developing a tonic vibration reflex. This was done in order to test how effectively the afferents were excited by the vibration, and to see if any insecurity in driving might be related to tremor.2. When the amplitude of vibration was 50 mum, and the tonic vibration reflex was reasonably well developed (> 1 N of active tension) all but one of forty-four Ia afferents were driven 1:1 by the vibration. Most were still driven by 30 mum vibration. The vibration, consisting of a train of discrete pulses at 150 Hz, was applied longitudinally in combination with a stretch of 1 mm to make the muscle taut.3. If the reflex was poorly developed (active tension < 1 N) the driving was on average less secure. However, fourteen of eighteen afferents then studied were still driven 1:1 by 50 mum vibration. The lower level of excitation by vibration was thought to be due to a deficiency of spontaneous fusimotor activity, because stroking the cat's tail or other similar gentle manipulation led each of the three misbehaving afferents so tested to be driven securely by 50 mum vibration; at the same time the reflex tension increased.4. Additional, indirect evidence favouring widespread security of Ia driving by 50 mum vibration in the presence of the reflex was obtained by modulating the amplitude of the 150 Hz vibration with a 7-10 Hz square wave and detecting any tension fluctuations at that frequency by spectral analysis. Small degrees of modulation (e.g. < 10%) produced little if any effect, although larger depths of modulation had a powerful action.5. When the amplitude of vibration was reduced to permit insecure driving but still to elicit a reflex response, the fluctuations in Ia firing pattern were unlike those previously seen in the de-efferented muscle. Spectral analysis showed that these firing fluctuations bore a general similarity to the tremor in the same preparation, but measurement of coherence demonstrated that the tremor and Ia firing were not well related. This was probably because individual Ia afferents were primarily influenced by local factors, and provides further evidence against the tremor of this preparation being attributable to the action of the stretch reflex.

Action Potentials↗
PubMed1982

Modulation of stretch reflexes during locomotion in the mesencephalic cat.

1. A cat preparation was used to study the modulation of stretch reflexes during locomotion. The brain stem was transected and locomotion was induced by electrical stimulation of the mesencephalic locomotor region below the level of transection. Three legs walked normally on a treadmill, while the fourth leg, which was denervated except for the soleus muscle, was held fixed. Brief length perturbations were applied to the soleus muscle at various phases of the stepping cycle.2. The stretch reflex in this muscle was deeply modulated during the step cycle, and reached its peak at or before the peak in soleus e.m.g. activity associated with the locomotion. A similar variation was observed when the sciatic nerve was stimulated electrically at a strength which elicited reflex activity (H wave), but did not directly elicit motor nerve activity (M wave). Variation in the reflex during electrical stimulation could not be accounted for by cyclic variation in fusimotor activity or in the afferent volley, but must be due to post-synaptic changes in alpha-motoneurones or in the presynaptic inputs to them.3. Large changes were also observed in intrinsic muscle stiffness during the step cycle. The maximum stiffness occurred near the time the limb would normally strike the ground during locomotion. A high stiffness would be useful in reducing the amount that the limb would yield under the weight of the body during the extension phase of the step cycle. The variation of the stretch reflex in parallel with stiffness suggests that reflexes could assist in this load compensation. The variation is not consistent with the idea that the stretch reflex is used to compensate for changes in intrinsic muscle properties, so that the total system behaves more like a spring of constant stiffness than does muscle alone.

Animals↗
PubMed1983

Effect of hypoxia on polysynaptic hind-limb reflexes of unanaesthetized fetal and new-born lambs.

Polysynaptic reflex excitation of the hind-limb flexor muscles was produced by stimulation of the distal peroneal or common sciatic nerve in ten fetal and six new-born unanaesthetized lambs. In the fetal lambs, stimulation at 1.1-3.25 times threshold for direct activation of the nerve produced a reflex response which was greatest during high-voltage electrocortical activity, being reduced 69 +/- 5.0% (mean +/- S.E. of the mean) during low-voltage electrocortical activity. Reduction of the fetal arterial PO2 from 23.3 +/- 0.58 to 12.2 +/- 0.4 mmHg caused a rapid and reversible reduction of the reflex in all fetal lambs with intact spinal cord, including two whose brain stems were transected at the level of the colliculi. Hypoxia did not alter the amplitude of the reflex in two fetuses in which the spinal cord at L1-2 had been cut several days previously. In new-born lambs reduction of the arterial PO2 to 43 mmHg did not change the reflex amplitude during episodes of quiet wakefulness or slow-wave sleep. Brief reduction of the Pa,O2 to 20-30 mmHg caused a statistically insignificant reduction of reflex amplitude during quiet sleep before behavioural arousal. It is concluded that hypoxaemia activates reticular mechanisms in the pons and medulla of fetal lambs which decrease the excitability of spinal reflex pathways. In the new-born lamb such effects are counteracted or prevented by mechanisms associated with arousal.

Animals↗
PubMed1995

Distribution of Ia effects onto human hand muscle motoneurones as revealed using an H reflex technique.

1. The possibility of eliciting H reflexes in relaxed hand muscles using a collision between the orthodromic impulses generated by magnetic cortical stimulation and the antidromic motor volley due to a supramaximal (SM) peripheral nerve stimulus was investigated in seven subjects. 2. Magnetic stimuli, applied through a circular coil (outer diameter, 13 cm) centred at the vertex, evoking EMG responses of 3-5 mV amplitude in the relaxed abductor digit minimi (ADM) muscle, and SM test stimuli to the ulnar nerve at the wrist producing a direct maximal motor response (Mmax) in the ADM muscle, were given either alone or combined. 3. In all subjects, combined cortical and SM ulnar stimulation produced a response after the Mmax with the latency of an H reflex evoked by the ulnar stimulus. This response occurred only within interstimulus intervals (1-20 ms) compatible with collision in the motor axons. The response behaved like an H reflex being time-locked to the SM ulnar stimulus, facilitated by voluntary activation of ADM muscle, depressed by vibration (4 s, 100 Hz) of ADM tendon and by a submotor-threshold ulnar nerve stimulus applied 50 and 80 ms before the combined stimulation, respectively. 4. In some subjects, it was also possible to distinguish an earlier response preceding the H reflex by 3 ms. Evidence is given that this response is probably of cortical origin. 5. Varying the intensity of magnetic stimulation resulted in a non-linear relationship between the H reflex size and the size of the cortical response. When the latter was between 5-25% of Mmax, H reflexes were small (2.5-7.5% of Mmax); with cortical responses between 25-50% of Mmax, there was a steep increase in H reflex amplitude (10-30% of Mmax). We suggest that this behaviour is due to an uneven distribution of Ia effects within the motoneurone pool.

Adult↗
PubMed2002

Excitability of human muscle afferents studied using threshold tracking of the H reflex.

In human peripheral nerves, physiological evidence has been presented for a number of biophysical differences between cutaneous afferents and alpha motor axons. The differences in strength-duration properties for cutaneous afferents and motor axons in the median nerve have been attributed to greater expression of a persistent Na(+) conductance (I(Na,P)) on cutaneous afferents. However, it is unclear whether the biophysical properties of human group Ia afferents differ from those of cutaneous afferents. The present studies were undertaken to determine whether the properties of human group Ia afferents can be studied indirectly using 'threshold tracking' to measure the excitability changes in the H reflex, and to determine whether the excitability of group Ia afferents differs from that of cutaneous afferents. The strength-duration properties of the soleus H reflex and soleus motor axons were measured at rest and during sustained voluntary contractions. Similar experiments were performed on the median nerve at the wrist to study the strength-duration properties of cutaneous afferents, alpha motor axons and H reflex of the thenar muscles. In addition, the technique of 'latent addition' was used to determine whether there was a difference in a low-threshold conductance on soleus Ia afferent and motor axons. The present findings indicate that the strength-duration time constant (tau(SD)) for the H reflex is longer than that for alpha motor axons, but similar to that for cutaneous afferents. There were no differences in tau(SD) for the soleus H reflex at rest and during contractions, suggesting that tau(SD) for the H reflex is largely unaffected by changes in synaptic or motoneurone properties. Finally, the difference in latent addition suggests that the longer tau(SD) of the soleus H reflex may indeed be due to greater activity of a persistent Na(+) conductance on Ia afferents than on soleus alpha motor axons.

Adult↗
PubMed2002

Stretch reflex gain in cat triceps surae muscles with compliant loads.

The triceps surae (TS) stretch reflex was measured in decerebrate cats during crossed extensor stimulation (tonic contractions) and after spinalization during rhythmic locomotor activity. The TS reflex force in response to a short pulse stretch measured during tonic contractions at low level of background activity was greater than when more background activity was present at the time of application of stretch. In contrast, the reflex force measured during rhythmic contractions was very small at low level of background force (flexion phase) and increased at moderate and high levels of background activity (extension phase). Thus, even in reduced preparations, a task modulation of the stretch reflex occurs. Throughout the experimental procedure, the torque motor used to stretch the muscles behaved like a spring of a preset compliance (from isometric to very compliant). A reflex model was used to simulate the responses obtained experimentally. The gain of the stretch reflex loop was estimated for each load condition and both behavioural tasks. The reflex loop gain was significantly larger as the compliance of the external load increased for both tonic and rhythmic contractions, although to a lesser extent in the phasically activated muscles. During rhythmic locomotor contractions the gain was less than 1, assuring stability of the system. In contrast, during tonic contractions against a compliant load the gain exceeded 1, consistent with the instability (oscillations, clonus) seen at times under these load conditions. However, the high gain and instability was only transient, since repeated stretch reduced the gain. Thus, non-linearities in the system assured vigorous responses at the onset of perturbations, but then weaker responses to ongoing perturbations to reduce the chance of feedback instability (clonus).

Animals↗
PubMed1972

Effect of voluntary contraction of the masseter and other muscles upon the masseteric reflex in man.

Experiments were carried out on seven adult subjects in order to establish the relationship between the magnitude of the masseteric reflex and the amount of voluntary activity present in the muscle at the time the reflex was evoked. At the same time, an effort was made to determine whether the magnitude of the reflex could be enhanced by the simultaneous voluntary contraction of muscles other than that being tested (the Jendrassik manoeuvre). The reflex was evoked by applying controlled downward thrusts to the mandible so as to produce a constant displacement in each case, and the response of the masseter muscle was recorded by means of small bipolar surface electrodes attached to the skin over the muscle. These responses were averaged by a computer in the presence of various static loads supported by the mandible. It was found that in all subjects the amplitude of the masseteric reflex appeared to increase as the weight supported by the mandible increased, and that in the majority of subjects it was possible to demonstrate Jendrassik facilitation by simultaneous contraction of the muscles of the upper limbs. The results of the experiments suggest that the enhancement of the masseteric reflex by voluntary contraction of the jaw-closing muscles may be due to autogenetic factors, synergistic factors, or both, and that at least two processes contribute to the amplitude of the masseteric reflex evoked by tooth contact during mastication-namely, the stimulation of muscle spindles by the impact of opposing teeth and facilitation caused by voluntary activity in the jaw-closing muscles before tooth contact.

Electric Stimulation↗
PubMed1976

Spinal reflexes and the concentrations of 5-HIAA, MHPG, and HVA in lumbar cereborspinal fluid after spinal lesions in man.

Descending bulbospinal pathways that employ specific neurotransmitter substances are known to be capable of modulating segmental reflex activity in the experimental animal. To determine whether this might also occur in man correlations have been sought between the activity in spinal reflex pathways and the lumbar cerebrospinal fluid (CSF) concentrations of 5-hydroxyindolacetic acid (5-HIAA), 3 methoxy-4-hydroxyphenylglycol (MHPG), and homovanillic acid (HVA) in 12 patients with complete or virtually complete spinal lesions. The concentrations of 5-HIAA and MHPG in lumbar CSF ARE REDUCED AFTER COMPLETE OR VIRTUALLY COMPLETE SPINAL LESIONS IN MAN. This may occur within 18 days of the lesion. MHPG concentrations appear to be inversely related to the level of the lesion. The HVA concentration in lumbar CSF is reduced when there is obstruction of the CSF pathways. No relationship could be demonstrated between the concentrations of 5-HIAA or MHPG in lumbar CSF and the activity in the spinal monosynaptic pathway (estimated from the proportion of the motoneurone pool activated by the Achilles tendon reflex or H reflex) or the activity of a spinal inhibitory mechanism (estimated by the degree of vibratory inhibition of the monosynaptic reflex). Patients with a tonic vibration reflex (TVR) tended to have higher MHPG levels. There appeared to be an association between low CSF HVA and enhanced vibratory inhibition of the monosynaptic reflex in the nine patients whose spinal lesions were complete.

Adolescent↗
PubMed1986

Brainstem reflexes and brainstem auditory evoked responses in Huntington's chorea.

Blink reflex, corneal reflex, jaw reflex, exteroceptive suppression in masseter muscles and brainstem auditory evoked potentials were measured in 20 patients with Huntington's chorea and 12 controls. A significantly increased latency of the second component of the homolateral and heterolateral blink reflex was found in the patient group as compared with the controls. The other investigations revealed no significant differences between patients and controls except for some facilitation of the jaw reflex in the patient group. Increase of second component latency of the blink reflex in the presence of normal corneal reflexes is suggestive of functional impulse conduction disturbance in the lower brainstem. It is discussed whether in Huntington's chorea this is to be attributed to alterations of cortical or striatal influence or to local brainstem abnormalities.

Adult↗
PubMed1992

Modulation of the soleus H-reflex during pedalling in normal humans and in patients with spinal spasticity.

Soleus H-reflexes were recorded in 10 normal subjects and seven patients with spasticity caused by incomplete spinal cord injury while they pedalled on a stationary bicycle which had been modified to trigger electrical stimuli to the tibial nerve at eight precise points in the pedal cycle. Stimulus strength was adjusted to yield M-waves of constant amplitude at each pedal position. During active pedalling, all normal subjects showed modulation of the H-reflex with the amplitude being increased during the downstroke portion of the pedal cycle and the reflex suppressed or absent during the upstroke. This modulation was not present during passive pedalling, with the experimenter cranking the pedals by hand, or when the pedals were locked at each of the eight positions. In five of the seven patients with spasticity, there was reduced or absent modulation of the H-reflex during active pedalling and the reflex remained large during pedal upstroke. It is concluded that descending motor commands that produce patterned voluntary activity during pedalling normally cause cyclical gating of spinal reflexes by either presynaptic or postsynaptic inhibitory mechanisms. Loss of supraspinal control over these spinal inhibitory systems could result in failure to produce appropriate suppression of reflexes during patterned voluntary movements such as pedalling or walking, and may be an important factor contributing to the functional disability in spasticity.

Adult↗
PubMed1993

Soleus H-reflex tests and clinical signs of the upper motor neuron syndrome.

Soleus H-reflex tests are used for elucidating pathophysiological mechanisms in motor control. The cumulative vibratory inhibition of the soleus H-reflex, the ratio of the reflex to direct muscle potential (H to M ratio) and the recovery curve of the soleus H-reflex were studied in 38 patients with varying signs of the upper motor neuron syndrome for a possible relation with clinical features. The results were compared with those obtained from a group of healthy volunteers. The magnitude of vibratory inhibition decreased with increase of hypertonia. The H to M ratio increased as the activity of the tendon reflex was enhanced and correlated to a lesser degree with muscle tone. Both the H to M ratio and late facilitation of the soleus H-reflex recovery curve were elevated in clonus. The findings suggest that alterations in the results of soleus H-reflex tests relate to specific clinical features of the upper motor neuron syndrome. Possible pathophysiological implications are discussed.

Adolescent↗
PubMed1996

Stapedial reflex in amyotrophic lateral sclerosis.

OBJECTIVE: To examine mechanisms controlling the stapedial reflex in patients with amyotrophic sclerosis (ALS). METHODS: The stapedial reflex was examined using impedance audiometry in 38 patients with sporadic ALS and in 25 age matched controls. RESULTS: All patients showed normal reflex decay test results. There were no significant differences between patients with ALS and control subjects in reflex threshold, latency, amplitude, or contraction time (C50). Although each reflex variable in the patients with classic or progressive muscular atrophy types of ALS showed no significant difference from that in control subjects, the patients with bulbar type ALS showed significantly longer latency, C50, and retraction time (D50), and significantly lower amplitude than control subjects. Three types of abnormal reflex waveforms (polyphasic, abnormally delayed retraction, and abnormally early retraction) were noted in six patients. CONCLUSION: The subclinical involvement of the stapedius motor neurons or of the supranuclear stapedius motor system might be responsible for the abnormalities of the stapedial reflex in ALS.

Acoustic Impedance Tests↗
PubMed1997

Altered pupillary size and darkness and light reflexes in Alzheimer's disease.

The purpose was to compare resting pupil diameter in darkness and light, and the pupillary darkness and light reflexes between a group of patients with Alzheimer's disease and a group of healthy old people. Nine medication free patients with Alzheimer's disease and nine healthy control subjects, matched for sex and age with the patients, participated. There were six men and three women and the median age was 72 years in both groups. Pupil diameter was monitored with an infrared television pupillometer. Resting pupil diameter was smaller in the Alzheimer's disease group (P = 0.041, in darkness). The amplitude and the maximum dilatation velocity of the darkness reflex were smaller for the Alzheimer's disease group (maximum dilatation velocity P < 0.002). The amplitude and the 75% recovery time of the light reflex response were reduced in the Alzheimer's disease group (P < 0.002 and P = 0.034 respectively). There was no difference in the latency of the reflex response between the two groups. The reduced pupil size and diminished darkness reflex in the Alzheimer's disease group are consistent with a sympathetic deficit in the patients. The reduction in light reflex amplitude and recovery time are likely to be secondary to the grossly diminished pupil size in the patient group. The lack of any change in light reflex latency in the patients with Alzheimer's disease argues against an afferent defect. The pupillary changes in patients with Alzheimer's disease are qualitatively the same as those seen in healthy old people and are consistent with the notion of "accelerated aging" in Alzheimer's disease.

Aged↗
PubMed2005

A 5-HT4 agonist, mosapride, enhances intrinsic rectorectal and rectoanal reflexes after removal of extrinsic nerves in guinea pigs.

Distension-evoked reflex of rectorectal (R-R) contractions and rectointernal anal sphincter (R-IAS) relaxations can be generated in guinea pigs through an extrinsic sacral excitatory neural pathway (pelvic nerves) as well as intrinsic cholinergic excitatory and nitrergic inhibitory pathways. The aim of the present study was to create intrinsic R-R and R-IAS reflex models by pithing (destruction of the lumbar and sacral cords; PITH) and to evaluate whether the prokinetic benzamide mosapride, a 5-HT(4) receptor agonist, enhances these reflexes. The mechanical activities of the R-R and R-IAS were recorded in the anesthetized guinea pig on days 2-9 after PITH. Although the basal rectal pressure at distension after PITH was significantly lower than control, the reflex indexes of R-R contractions and synchronous R-IAS relaxations were unchanged between days 4 and 9 after PITH. The frequency of spontaneous rectal and IAS motility were also unchanged. Immunohistochemical studies revealed that the distribution of myenteric and intramuscular interstitial cells of Cajal (ICC) were not altered after PITH. Mosapride (0.1-1.0 mg/kg iv) dose-dependently increased both intrinsic R-R (maximum: 1.82) and R-IAS reflex indexes (maximum: 2.76) from control (1.0) 6-9 days after PITH. The 5-HT(4) receptor antagonist, GR-113808 (1.0 mg/kg iv) decreased the R-R and R-IAS reflex indexes by approximately 50% and antagonized the effect of mosapride (1.0 mg/kg iv). The present results indicate that mosapride moderately enhanced intrinsic R-R and R-IAS reflexes functionally compensated after deprivation of extrinsic nerves, mediated through endogenously active intrinsic 5-HT(4) receptors.

Acute Disease↗
PubMed1979

Ascending spinal pathways for the somatosympathetic A and C reflexes.

Sympathetic unit nerve activity was recorded from the cervical sympathetic trunk of anesthetized, vagotomized, and carotid sinus-denervated cats. Single pulses or short trains of pulses with suprathreshold intensity for afferent C fibers were applied to the common peroneal nerve to elicit somatosympathetic A or simultaneous A and C reflexes, respectively. Bilateral lesions of an ascending spinal pathway in the dorsolateral sulcus area (DLS) of the T12 spinal cord eliminated the sympathetic C reflex. The remaining somatosympathetic A reflex was abolished by the subsequent lesion of an ascending spinal pathway in the dorsolateral funiculus (DLF) bilaterally. The duration of the silent period was found to be mainly influenced by an ascending spinal pathway in the DLF. It was concluded that excitation of afferent A fibers activates ascending pathways mainly in the DLF and a smaller contribution in the DLS of the spinal cord. This produces a small reflex excitation (A reflex) followed by a long silent period in the sympathetic nerves resulting in a net decrease of total sympathetic nerve activity. On the other hand, excitation of afferent C fibers activates the ascending spinal pathway in the DLS. This produces a reflex excitation (C reflex) which exceeds the decreased activity due to the silent period resulting in a net increase of total sympathetic nerve activity.

Action Potentials↗
PubMed1984

Baroreceptor heart rate reflex in rabbits after reversal of renal hypertension.

The baroreceptor heart rate reflex was studied in 26 renal hypertensive and 17 normotensive rabbits and in 13 rabbits after reversal of hypertension. Blood pressure and heart rate were varied by bolus injections of methoxamine or nitroglycerine in conscious rabbits, and a logistic sigmoid function was fitted to the pressure-heart rate data to estimate upper and lower heart rate plateaus, median blood pressure, and reflex sensitivity. In hypertensive rabbits, resetting of the blood pressure-heart rate curves to higher pressures was associated with reduced reflex sensitivity, increased lower heart rate plateau, and operating pressures significantly greater than the region of maximum reflex sensitivity, resulting in substantial attenuation of reflex bradycardia in response to hypertension. Reflex bradycardia after smoke inhalation was normal. Reversing the hypertension completely reversed the abnormalities in baroreflex control of heart rate. Renal hypertension in the rabbit produces functional changes that are completely reversible in the baroreceptor-heart rate reflex, probably in the afferent limb.

Animals↗
PubMed1995

Cardiac-cardiovascular reflexes induced by hydrogen peroxide in cats.

We have shown previously that reactive oxygen species stimulate abdominal sympathetic afferents to cause reflex cardiovascular activation. Because myocardial ischemia and reperfusion also generate reactive oxygen species, we investigated the possibility that cardiovascular reflexes could be induced by topical application of H2O2 to the anterior or posterior ventricular surface in cats anesthetized with alpha-chloralose. Mean arterial pressure (MAP), heart rate (HR), left ventricular (LV) pressure, aortic flow (AF), and first derivative of LV pressure at 40 mmHg developed pressure (LV dP/dt40) were monitored. H2O2 (44 and 130 mumol) significantly increased MAP but not HR or LV dP/dt40 in intact cats (n = 8). Application of H2O2 (44 mumol) significantly increased MAP (129 +/- 9 to 152 +/- 10 mmHg), HR (240 +/- 11 to 245 +/- 10 beats/min), AF (191 +/- 13 to 212 +/- 17 ml/min), total peripheral resistance (0.68 +/- 0.13 to 0.73 +/- 0.04 peripheral resistance units), and LV dP/dt40 (2,666 +/- 145 to 3,012 +/- 205 mmHg/s) after bilateral cervical vagotomy (n = 6). These H2O2-induced excitatory responses were abolished after bilateral T1-T4 ganglionectomy. In six additional cats, H2O2 (44 mumol) significantly decreased MAP (114 +/- 5 to 102 +/- 5 mmHg), HR (207 +/- 7 to 190 +/- 7 beats/min), and LV dP/dt40 (2,776 +/- 168 to 2,600 +/- 153 mmHg/s) after sympathectomy. These depressor responses were eliminated after vagotomy. The magnitude of the cardiovascular reflexes was increased or decreased in a dose-dependent fashion in vagotomized or sympathectomized cats, respectively, over a range of 440 nmol to 44 mumol H2O2. Application of H2O2 to the anterior or posterior ventricular surface resulted in similar pressor or depressor reflexes. Dimethylthiourea and deferoxamine abolished pressor or depressor responses evoked by H2O2 in both vagotomized (n = 8) and sympathectomized (n = 8) cats. We conclude that reactive oxygen species, particularly the hydroxyl radical, can participate in activating cardiac afferents responsible for reflex cardiovascular responses during myocardial ischemia and reperfusion. An inhibitory reflex is transmitted through vagal afferents, whereas an excitatory reflex is conducted by sympathetic cardiac afferents.

Animals↗